I have recently removed a bradford pear tree, and now have a problem with new growth coming up in my lawn from the root of the removed tree. The tree was about 11 years old.
How do I stop this root growth?

You will have to dig the root up to stop the growth.

Paint the stump with an herbicide - Round Up, brush killer, or even weed killer. If it still sprouts, apply it to the sprouts. A stump grinder is easier than digging.